Technology trends moving toward high frequency products have traditionally required the use of flipchip packaging. The use of flipchip packaging comes at the price of increased cost and higher processing complexity. If wirebond package configurations could be improved while maintaining the cost benefits over flipchip packages, several high frequency products designed for flipchip implementation could be packaged using wirebond alternatives. In particular, the effect of inductance of the conductive lines in wirebond packages significantly interferes with meeting return loss specifications in several high frequency applications.
Return loss measures the amount of power that is lost due to reflections and mismatch loss in the conductive line. Return loss is caused by impedance mismatches between the conductive lines (i.e. bondwires) and the bonding pads and the effect impedance mismatches has on signals traversing through the characteristic impedance of the conductive line. The termination of the conductive lines also plays a significant role in determining the return loss on the conductive line, when the characteristic impedance of any part of the conductive line relative to the value of the termination is mismatched. There are two types of return loss: input return loss and output return loss. Signals that are ingress to the integrated circuit (i.e. a receiver circuit) are denoted as having input return loss. Similarly, egress circuits are denoted as having an output return loss. The invention described herein is applicable to both forms of return loss.
Previous methods to reduce return loss due to inductance of wirebonds focused on adding multiple conductive lines in parallel. These methods are hampered by mutual inductance between conductive lines, which limits improvement in the overall inductance. As signal frequencies increase, return loss due to inductance of the signal line becomes a significant problem to overcome.
As a result, there is a need to solve the problems of the prior art to provide a method and apparatus for reducing the return loss of a wirebond package.